[Bug libstdc++/52839] double free or corruption running tr1/.../default_weaktoshared.exe

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52839

--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-04-10 15:36:40 UTC ---
You're quite right, my apologies for telling you that wouldn't happen.

In bits/shared_ptr_base.h we have:

  template<> 
    inline void
    _Sp_counted_base<_S_atomic>::
    _M_add_ref_lock()
    {
      // Perform lock-free add-if-not-zero operation.
      _Atomic_word __count = _M_use_count;
      do
    {
      if (__count == 0)
        __throw_bad_weak_ptr();
      // Replace the current counter value with the old value + 1, as
      // long as it's not changed meanwhile. 
    }
      while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
                      true, __ATOMIC_ACQ_REL, 
                      __ATOMIC_RELAXED));
    }


This will always be atomic if __default_lock_policy is _S_atomic

The problem is in ext/concurrence.h:

  // Compile time constant that indicates prefered locking policy in
  // the current configuration.
  static const _Lock_policy __default_lock_policy = 
#ifdef __GTHREADS
#if (defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2) \
     && defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4))
  _S_atomic;
#else
  _S_mutex;
#endif
#else
  _S_single;
#endif

This is inconsistent with the definition of _GLIBCXX_ATOMIC_BUILTINS and means
that on power shared_ptr thinks it can use atomics, but the "dispatch"
functions in ext/atomicity.h don't use atomics.


__default_lock_policy should not say to use atomics if the dispatch functions
don't use atomics.  

A quick fix would be to only set __default_lock_policy=_S_atomic if
_GLIBCXX_ATOMIC_BUILTINS is also defined